In weighing cells that function according to the principle of electromagnetic force compensation and which are also referred to as magnetic force restoration (“MFR”) weighing cells, the weight force of the weighing object is transferred either directly or by way of one or more force-transmitting levers to an electromechanical measurement transducer which generates a compensation force corresponding to the weight force of the weighing object and at the same time delivers an electrical signal which is further processed by an electronic weighing module in the processor unit and indicated on a display panel.
In a MFR weighing cell, the weight force of the weighing object is measured by way of an electrical substitute quantity. For a variety of technical reasons, this measurement is subject to inaccuracies, and the relative measurement resolution of a MFR weighing cell is therefore limited. A MFR weighing cell is further limited in its relative measurement resolution because it has a balance beam which can be caused to resonate to a greater or lesser degree by ground vibrations. Such vibrations can manifest themselves in the weighing signal as disturbances of a kind that cannot be compensated.
According to a known concept of the state of the art which is used in high-resolution force-measuring devices such as, e.g. mass comparators, the limits of the high-resolution range which are inherent in the electrical measurement process are shifted in discrete steps by initially overloading the balance beam on the side of the compensation force, i.e. the side of the measurement transducer and then adding so-called substitution weights to the opposite side, i.e. the side of the pan hanger, in order to establish equilibrium. The function of these substitution weights is to shift the measurement window of the force-measuring device which, in the absence of substitution weights, would be confined between a minimum- and a maximum weighing load, wherein the shifting of the measurement window occurs in discrete amounts equal to the values of the substitution weights. Force-measuring devices of this type are also referred to as window-comparator weighing cells, and state-of-the-art embodiments are described, for example, in DE 2, 621,483 B1, which has an equivalent in U.S. Pat. No. 4,153,124.
Within the realm of gravimetric measuring instruments with electromagnetic force compensation, the measurement window of a force-measuring device is the weight range within which the mass of the weighing object can be measured by varying the compensation force of the measurement transducer. The width of this weight range is thus defined and limited by the maximum compensation force that can be generated by the measurement transducer (the stronger the compensation force of a measurement transducer, the wider the measurement window).
A balance disclosed in U.S. Pat. No. 4,165,791 illustrates how the mechanical zero point, and with it the measurement window, can be shifted in a force-measuring device. In the process of measuring an unknown weight, all of the substitution weights are at first resting on the pan hanger, and the equilibrium in this initial zero position of the balance is maintained by a counterweight. After the weighing object has been set on the balance, an amount of weight that is just short of the weight of the weighing object is removed from the balance hanger. The remaining imbalance is compensated by an electromagnetic coil. This concept has the disadvantage that the balance beam permanently carries a large amount of mass, which reduces the mechanical stability of the weighing cell against ground vibrations, in particular against rotatory resonances. As a way of reducing the complexity of the design, the number of substitution weights in a balance of the kind disclosed in U.S. Pat. No. 4,165,791 is kept to a minimum. This has the consequence that the measurement window can only be shifted in large jumps.
In a balance disclosed in DE 2 803 978 A1, the total effect of all weights that are suspended from the scale mechanism is compensated by a counterweight which is arranged to the coil lever at the opposite end of where the pan hanger is attached. Taking into account the lever ratio of the coil lever, the counterweight is designed to hold equilibrium with the largest weight that can be measured on the balance, with the dead weight of the pan carrier and the weighing pan included. The counterweight is set in place after the final assembly of the weighing cell, and the final adjustment of the weighing mechanism is performed by means of an adjustment screw. After the counterweight has been adjusted in this manner, it is normally locked in place and sealed by the manufacturer, for example with a drop of thread-locking adhesive, to prevent the counterweight from coming loose or being moved.
The mechanical zero point is the operating point of a force-measuring device where the balance beam is in equilibrium without a compensation force acting on it. This is the condition where the measurement error of the electrical measurement quantity is smallest, meaning that the force-measuring device has its highest measurement resolution around this point. Furthermore, at this operating point the force-measuring device is insensitive to vertically directed ground disturbances.
According to a solution that is disclosed in DE 103 42 272 B3 (an equivalent is found in U.S. Pat. No. 7,780,579), wherein a dead load is compensated by a counterweight, the resonance tendencies of the transmission lever are minimized by arranging the counterweight so that alternating coil forces are not acting in a way that excites oscillations. This is achieved by placing the counterweight at a location above the coil. Thus, by establishing a standing-wave node within the coil, the tendency to excite the transmission lever into resonance is removed. The counterweight is in this case of a fixed amount, designed to counterbalance the dead load, i.e. the weighing pan, with no provisions to allow for a change in the dead load, for example to take a container into account as additional dead load.
A balance with a sliding weight is shown in GB 2 000 305 A. However, this balance is not based on the operating principle of electromagnetic force compensation, but uses the sliding weight as a means of weighing an object placed on the balance pan. An equilibrium detector consisting of a flexible leaf carrying four strain gauges measures the deflection of the balance beam. In response to the measured deflection, an electrical spindle drive mechanism moves the sliding weight along the balance beam in order to restore the equilibrium of the balance beam. The revolutions of the spindle are picked up with a decoder to determine the position of the sliding weight. Finally the weight of the object is calculated based on the position of the sliding weight.
The present invention has the objective to provide a force-measuring device in which the mechanical zero point and the measurement window can be adapted automatically and with continuous variability to the requirements of the load that needs to be measured, while keeping the applied load on the balance beam to a minimum.